One of the largest oil spills in recent southern Californian history fouled popular beaches and killed wildlife, as crews rushed on Sunday to contain the crude before it spread further into protected wetlands.
At least 573,000 litres of oil spilled into the waters off Orange County, the city of Huntington Beach said.
“The spill has significantly affected Huntington Beach, with substantial ecological impacts occurring at the beach and at the Huntington Beach Wetlands,” the city said.
The oil created a kilometres-wide sheen in the ocean and washed ashore in sticky, black globules along with dead birds and fish.
Crews led by the US Coast Guard used skimmers and floating barriers called booms to try to stop further incursion into the wetlands and the Bolsa Chica Ecological Reserve.
The closure stretched from the Huntington Beach Pier nearly 6.5 kilometres south to the Santa Ana River jetty, amid summer-like weather that would have brought many beachgoers to the wide strand.
Officials cancelled the final day of the annual Pacific Air Show, which typically draws thousands of spectators to Huntington Beach, a city of about 199,000 residents 50km south of downtown Los Angeles.
The show featured flyovers by the US Navy Blue Angels and the US Air Force Thunderbirds.
The oil slick came from a broken pipeline connected to an offshore platform known as Elly, Orange County Supervisor Katrina Foley said on Twitter.
Ms Foley said Newport Beach Mayor Brad Avery told her that he encountered the oil slick while in a boat travelling back to the mainland from Santa Catalina Island.
“He saw dolphins swimming through the oil,” she tweeted.
“While the leak has not been completely stopped, preliminary patching has been completed to repair the oil spill site,” with additional repairs planned, the city said early on Sunday.
On February 7, 1990, the oil tanker American Trader ran over its anchor off Huntington Beach, spilling nearly 1.6 million litres of crude. Fish and about 3,400 birds were killed.
In 2015, a ruptured pipeline north of Santa Barbara sent 541,313 litres of crude oil on to Refugio State Beach.
On Saturday night, Orange County officials expressed concern about the environmental effects of the spill and hoped crews could stop the oil before it flowed into sensitive wetlands.
“We’ve been working with our federal, state and county partners to mitigate the impact that could be a potential ecological disaster,” Huntington Beach Mayor Kim Carr said.
Huntington Beach is home to birds including gulls, willet, long-billed fletchers, elegant teens and reddish egrets, which are a rarity on the west coast, said Ben Smith, a biologist and environmental consultant for Orange County.
Mr Smith drove to the beach Sunday to observe wildlife ahead of a construction project planned at the mouth of the Santa Ana River, which flows into the ocean at the border of Huntington State Beach and Newport Beach.
“There’s tar everywhere,” Mr Smith told the Los Angeles Times. “You think by now we would have figured out how to keep this kind of thing from happening, but I guess not.”